The fundamental notion in Postgres is that of a class, which is a named
collection of object instances. Each instance has the same
collection of named attributes, and each attribute is of a specific
type. Furthermore, each instance has a permanent object identifier (OID) that is unique throughout the installation.
Because SQL syntax refers to tables,
we will use the terms table and class interchangeably. Likewise, an SQLrow is an instance and SQLcolumns are attributes. As previously discussed, classes are
grouped into databases, and a collection of databases managed by a
single postmaster process
constitutes an installation or site.